I have leased a few times with different arrangements. First off, I assume you are looking at a full lease where the other person will have full-time access to your horse? In that case, the lease should cover all of your expenses. If you are looking at a part lease, where you still ride your horse, then you will need to set the ground rules of how much time the other person gets and when. Also, the cost should be proportional. As a benchmark, you could check into what it costs per ride for someone to ride a leased horse in a lesson.
It is vitally important to draw up a lease contract and include the particulars of where and when the other person can take your horse, as well as who is responsible if things go badly. Can they trailer the horse to events or trail rides? Who is responsible for calling the vet in an emergency? Who pays the vet bills? Who is responsible for calling the farrier, vaccinations, etc.? Is there insurance to cover injury, illness, mortality? What if the horse is injured and cannot be ridden for a period of time during the lease? What if the injury is an accident, and what if it is caused by negligence?
I recommend doing an online search for a few different lease agreements and carefully looking over their contents. Alternatively, your local horse club, or a local barn might have a standard agreement they would let you look at. I also recommend a thorough health exam before starting the lease and making sure vaccinations, etc are all up to date. It is worthwhile for both parties to have that piece of mind before engaging in a lease contract, and it is in the best interests of your horse.
Previously when I have leased my horses, it worked out well. It did seem like a lot of work to set up the lease and attend to all of the details, but I believe it payed off in that I had no problems and everyone was happy, especially my horses.